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 down very little water, and none at all for the rest of the year. Unique in this respect among the great rivers of the globe, the Nile seems for the greater part of its course to be a river destitute of tributary basins. On its west bank nothing occurs for 2,200 miles from its mouth except some wadics flushed during the ruins.

But then follows a sudden and remarkable contrast, due to the changed climatic conditions. All the triangular region comprised between the Bahr-el-Jebel, the Nile, and Congo water-parting, and the Dar-For uplands, is intersected by numerous perennial streams nearly converging in the direction of the old lacustrine basin now filled with alluvia and vegetable refuse. With their minor headstreams and affluents they form a vast and intricate hydrographic system, extremely difficult accurately to survey, especially owing to the varied and shifting nomenclature. Like the Nile itself, every secondary branch bears as many names as there are tribes in its valley or neighbourhood. The most important appear to be the Yei, which is lost in the swamps bordering the left bank of the Nile; the Rol, flowing to the Bahr-el-Ghazal; the Roa and Tonj, whose united waters form the Apabu; the Diur, which reaches the Bahr-el-Ghazal near Meshra-er-Rek, and which is the most copious of its many affluents; the Pango, a branch of the Diur; lastly the Famikam, better known as the Bahr-el-Arab, which forms the northern limit of the whole region, and which, after its junction with the Ghazal, deflects the Nile eastwards.

Most of these streams have a very gentle incline, the most rapid being those that take their rise in the mountains near lake Albert Nyanza. Some have their source altogether in the plains, offering an almost imperceptible transition to the basin of the Congo. In their lower course the Rol, Diur, and some others have too slight a fall to scour their beds of the vegetation constantly accumulating. The consequence is that, like the Nile, they overflow their banks, during the floods converting the whole country for some thousiinds of square miles into an impassable morass. A large portion of the rainfall in this part of the Nile basin evaporates before reaching the main stream. Here the annual rains represent a volume greater than the whole discharge of the Nile at Cairo.

At the point where it resumes its normal northerly course beyond the region of sedd, the Nile is joined on its east bank by the Sobat, which is also known by a great variety of names. The Sobat, which drains a very large area, and which